Bulk acoustic wave devices, which can be used as resonators for radio frequency (rf) filters, have heretofore been formed as discrete components due to their incompatibility with conventional complementary-metal oxide-semiconductor (CMOS) fabrication processes. It would be very desirable in terms of reduced cost and size to be able to fabricate a bulk acoustic resonator directly on a silicon or silicon-on-insulator (SOI) substrate.
Efforts to do this have been underway for several years to produce film bulk acoustic resonators (FBARs) which are compatible with CMOS processing (see e.g. R. Aigner, “Volume Manufacturing of BAW-Filters in a CMOS Fab,” Proceedings of the 2nd International Symposium on Acoustic Devices for Future Mobile Communication Systems, Chiba University, Japan, pp. 129-134, Mar. 3-5, 2004). However, such FBARs are limited in that multiple resonators operating at different frequencies cannot easily be fabricated on a common substrate since the film thickness of the piezoelectric material sets the operating frequency so that all the FBARs formed on a common substrate generally have the same operating frequency.
More recently, contour mode aluminum nitride (AlN) resonators have been reported that utilize the d31 piezoelectric coefficient, which provides an in-plane lateral displacement, for electrical drive and sense (see e.g. G. Piazza et al., “Dry-Released Post-CMOS Compatible Contour-Mode Aluminum Nitride Micromechanical Resonators for VHF Applications,” Proceedings of the 2004 Solid-State Sensor, Actuator and Microsystems Workshop, Hilton Head Island, S.C., pp. 37-40, 2004; G. Piazza et al., “Low Motional Resistance Ring-Shaped Contour-Mode Aluminum Nitride Piezoelectric Micromechanical Resonators for UHF Applications,” Proceedings of the 18th IEEE International Conference on Micro Electro Mechanical Systems 2005, pp. 20-23, Jan. 30-Feb. 3, 2005). The use of contour modes, with frequencies determined by in-plane dimensions rather than by layer thickness, enables the fabrication of multiple resonators with different frequencies on a common substrate. However, these contour mode resonators are supported by tethers anchored to the substrate. The tethers provide relatively poor heat sinking to the substrate, and this limits the power handling capability of these contour mode resonators. Additionally, the tethers provide a discontinuity which can distort the shape of the contour mode and produce spurious modes even when notching about the tethers is used. These effects produced by the tethers become more pronounced as the size of the contour mode resonators is reduced to increase the operating frequency.
The present invention can overcome many of the limitations of the prior art by providing a MEM resonator having an acoustic resonator which is suspended from a substrate by an acoustic reflector. The acoustic reflector can provide an increased heat sinking capability for the device, while providing acoustic isolation from the substrate. The acoustic reflector can also support the acoustic resonator over a relatively large area without distorting the shape of a contour mode therein.
These and other advantages of the present invention will become evident to those skilled in the art.